1. Field of the Invention
The present invention relates generally to asynchronous Wideband Code Division Multiple Access (WCDMA) communication. In particular, the present invention relates to a method and apparatus for efficiently scheduling an enhanced dedicated channel for uplink packet transmission, used by user equipment (UE) located in a soft handover region.
2. Description of the Related Art
A Universal Mobile Telecommunications Service (UMTS) system which is a 3rd generation mobile communication system that is based on Global System for Mobile Communications system (GSM) which is a European mobile communication system and uses Wideband Code Division Multiple Access (WCDMA), provides a consistent service capable of transmitting packet-based text, digitized audio or video, and multimedia data at a high rate of 2 Mbps or higher no matter where mobile phone users or computer users are located. UMTS uses the concept of virtual access called “packet-switched access” that uses a packet protocol like Internet Protocol (IP), and can always access any terminal in the network.
FIG. 1 is a diagram illustrating a configuration of a conventional UMTS Terrestrial Radio Access Network (UTRAN) in a UMTS system. Referring to FIG. 1, a UTRAN 12 comprises radio network controllers (RNCs) 16a and 16b, and Node Bs 18a, 18b, 18c and 18d, and connects a terminal or user equipment (UE) 20 to a core network 10. Each of the Node Bs 18a, 18b, 18c and 18d can have a plurality of cells in its lower layer. The RNCs 16a and 16b each control their associated Node Bs in their lower layers. For example, in FIG. 1, the RNC 16a controls the Node Bs 18a and 18b, and the RNC 16b controls the Node Bs 18c and 18d. The Node Bs 18a, 18b, 18c and 18d each control their associated cells. One RNC and its associated Node Bs and cells controlled by the RNC constitute a radio network subsystem (RNS) 14a or 14b. 
Each of the RNCs 14a and 14b assigns or manages radio resources of its Node Bs 18a to 18d, and each of the Node Bs 18a to 18d provides the radio resources. The radio resources are generated per cell, and the radio resources provided by the Node Bs 18a to 18d refer to radio resources of cells managed by the Node Bs. The UE 20 can create a radio channel using a radio resource provided by a particular cell of a particular Node B, and perform communication using the created channel. Because distinguishing or differentiating between Node Bs 18a to 18d and their associated cells is meaningless to the UE 20 and the UE 20 recognizes only the physical layers created per cell, the terms “Node Bs 18a to 18d” and “cells” will be used herein as having the same meaning.
An interface between the UE 20 and RNCs 16a and 16b is called a Uu interface, and its detailed hierarchical structure is illustrated in FIG. 2. The Uu interface is divided into a control plane used for control signal exchange between the UE 20 and the RNCs 16a and 16b and a user plane used for data transmission.
Referring to FIG. 2, control-plane (C-plane) signaling 30 is processed through a radio resource control (RRC) layer 34, a radio link control (RLC) layer 40, a media access control (MAC) layer 42, and a physical (PHY) layer 44. A user-plane (U-plane) information 32 is processed through a packet data control protocol (PDCP) layer 36, a broadcast/multicast control (BMC) layer 38, the RLC layer 40, the MAC layer 42 and the PHY layer 44. Among the layers illustrated herein, the PHY layer 44 is located in each cell and the MAC layer 42 through the RRC layer 34 are located in a RNC.
The PHY layer 44 provides an information transfer service using a radio transfer technique, and corresponds to Layer 1 of the Opening Systems Interconnection (OSI) model. Connection between the PHY layer 44 and the MAC layer 42 is achieved by transport channels, and the transport channels are defined according to how specific data is processed in the PHY layer 44.
The MAC layer 42 is connected to the RLC layer 40 through logical channels. The MAC layer 42 delivers data received through a logical channel from the RLC layer 40 to the PHY layer 44 through a proper transport channel, and delivers data received through a transport channel from the PHY layer 44 to the RLC layer 40 through a proper logical channel. In addition, the MAC layer 42 inserts additional information into data received through a logical channel or a transport channel, or analyzes additional information inserted into data and performs an appropriate operation according to the analyzed additional information. Further, the MAC layer 42 controls a random access operation. In the MAC layer 42, a part related to the user plane is called MAC-d, and a part related to the control plane is called MAC-c.
The RLC layer 40 manages setup and release of a logical channel. The RLC layer 40 can operate in one of three operation modes of an acknowledged mode (AM), an unacknowledged mode (UM) and a transparent mode (TM), and each operation mode provides a different function. Generally, the RLC layer 40 has a function of disassembling or assembling a service data unit (SDU) provided from an upper layer in an appropriate size, and an error correction function.
The PDCP layer 36 is located in an upper layer of the RLC layer 40 in the user plane, and has a function of compressing and decompressing a header of data transmitted in the form of an IP packet and a function of losslessly-transmitting data in a situation where a RNC providing a mobile service to a particular UE is changed.
A characteristic of the transport channels connecting the PHY layer 44 to its upper layers is determined by a transport format (TF) that defines physical layer processing processes, such as convolutional channel encoding, interleaving and service-specific rate matching.
Particularly, a UMTS system uses an enhanced uplink dedicated channel (E-DCH) so as to enhance packet transmission performance in uplink communication from a UE to a Node B (or base station (BS)). In order to support stabilized high-speed data transmission, the E-DCH supports such techniques as Hybrid Automatic Retransmission Request (HARQ) and Node B-controlled scheduling. Processing of the E-DCH is achieved by a MAC-e layer located in a lower layer of a MAC-d layer, and in the MAC-e layer, E-DCH data with control information added thereto is called a MAC-enhanced Protocol Data Unit (MAC-e PDU).
FIG. 3 is a diagram illustrating a conventional method of transmitting uplink packet data over an E-DCH in a radio uplink comprising channels 111, 112, 113 and 114. Referring to FIG. 3, reference numeral 100 represents a Node B supporting the E-DCH, and reference numerals 101, 102, 103 and 104 represent UEs transmitting the E-DCH. The Node B 100 analyzes conditions of the UEs 101 through 104 that use the E-DCH, and schedules a data rate of the UEs 101 through 104 according to the analysis result. In order to increase the entire system performance, the scheduling is performed in such a manner that a UE located farther from a Node B is assigned a lower data rate and a UE located nearer to the Node B is assigned a higher data rate as long as a measured Rise-over-Thermal (RoT) value of the Node B does not exceed a target RoT value.
FIG. 4 is a signaling diagram illustrating a conventional procedure for transmitting and receiving messages over an E-DCH. Referring to FIG. 4, in step 202, a Node B and a UE set up an E-DCH therebetween. The E-DCH setup process 202 comprises a process of transmitting messages through a dedicated transport channel. After the E-DCH setup, the UE provides scheduling information to the Node B in step 204. The scheduling information can include UE's transmission power information representing uplink channel information, information on available extra power of the UE, and the amount of transmission data stored in a UE's buffer.
In step 206, the Node B, which receives scheduling information from a plurality of UEs in communication with the Node B, monitors the scheduling information received from the plurality of UEs in order to schedule a data rate of each UE. Specifically, in step 208, the Node B allows the UE to transmit an uplink packet and transmits scheduling assignment information to the UE. The scheduling assignment information comprises a granted maximum data rate and granted transmission timing.
In step 210, the UE determines a transport format (TF) of the E-DCH to be transmitted in a reverse direction, using the scheduling assignment information. The UE transmits uplink (UL) packet data over the E-DCH in step 212, and at the same time, transmits the TF information to the Node B in step 214. In step 216, the Node B determines whether there is an error in the TF information and the packet data. In step 218, the Node B transmits a non-acknowledge (NACK) to the UE over an ACK/NACK channel if there is an error in any of them. However, if there is no error in both of them, the Node B transmits an acknowledge (ACK) to the UE through the ACK/NAKC channel. If the ACK is transmitted, transmission of the corresponding packet data is completed and thus, the UE transmits new data through the E-DCH. However, if the NACK is transmitted, the UE retransmits the same packet data over the E-DCH.
The E-DCH, as it is an enhanced channel for uplink packet transmission, has the basic characteristics of a dedicated channel, and one of the characteristics is to support soft handover. A UE located in a soft handover region can receive downlink information from all of Node Bs included in its active set. Therefore, the UE located in the soft handover region receives scheduling assignment information from all of the Node Bs included in the active set in order to transmit the E-DCH. As a result, because the UE receives different scheduling assignment information from the Node Bs included in the active set, the UE needs to determine whether to transmit the E-DCH according to the different scheduling assignment information.
As described above, in the conventional communication system supporting the E-DCH, a UE located in a soft handover region is scheduled in such a manner that all Node Bs included in the active set transmit scheduling assignment information to the UE, causing an overhead problem in terms of code resources or power transmission resources. In addition, the UE receiving the different scheduling assignment information has difficulty in determining whether to transmit the E-DCH.